Intra-aortic balloon pump therapy is frequently prescribed for patients who have suffered a heart attack or some other form of heart failure. In such therapy, a thin balloon is inserted through an artery into the patient's aorta. The balloon is connected through a series of tubes to a complex drive apparatus which causes the balloon to inflate and deflate repeatedly in time with the patient's heartbeat, thereby removing some of the load from the heart and increasing blood supply to the heart muscle during the therapy period.
The inflation/deflation apparatus supplies positive pressure for expanding the balloon during an inflation cycle and negative pressure for contracting the balloon during a deflation cycle. In a conventional prior art apparatus shown schematically in FIG. 1, an intra-aortic balloon 10 is surgically inserted into a patient's aorta and is connected through a catheter 12 having a small diameter lumen and an extender 14 having a relatively large diameter lumen to an isolator 18 divided by a pliant membrane 20 into a primary side 22 and a secondary side 24. The entire volume between membrane 20 and balloon 10 is completely filled with a gas, such as helium, supplied by a gas source 26. A positive pressure source 28 is connected through a solenoid valve 30 to the input or primary side 22 of isolator 18. Similarly, a negative pressure source 32 is connected through a solenoid valve 34 to the input or primary side 22 of isolator 18. The primary side 22 of isolator 18 is also connected through a solenoid valve 36 to a vent or exhaust port 38. Typically in such systems, the isolator, gas source, negative and positive pressure sources, vent port and their associated valves together comprise a reusable drive unit, and the extender, catheter and balloon are disposable so as to accommodate sterility concerns.
During an inflation cycle, solenoid valve 30 is opened to permit positive pressure from positive pressure source 28 to enter primary side 22 of isolator 18. This positive pressure causes membrane 20 to move toward secondary side 24, thereby forcing the helium in the secondary side to travel toward and inflate balloon 10. For deflation, solenoid valve 30 is closed and solenoid valve 36 is opened briefly to vent the gas from primary side 22, after which valve 36 is closed. Solenoid valve 34 is then opened, whereupon negative pressure source 32 creates a negative pressure on the primary side 22 of isolator 18. This negative pressure pulls membrane 20 toward primary side 22, whereby the helium is drawn out from the balloon.
It is desirable in intra-aortic balloon pump therapy to inflate and deflate the balloon as rapidly as possible. Rapid cycling would permit the therapy to be performed more effectively, and would enable smaller diameter catheters to be used, thereby reducing the possibility of limb ischemia. Although the prior art system described above permits rapid inflation and deflation cycles, the configuration of this system creates inherent limitations in the cycle speed which can be achieved.
Thus, in a typical inflation cycle, pressurized gas from positive pressure source 28, at an initial pressure of about 8 psi, is used to inflate balloon 10 to an end inflation pressure of about 2 psi, which is about the blood pressure of a normal patient. (In the present specification, all references to psi, unless otherwise noted, are to gauge pressures, not absolute pressures.) In the initial portion of the inflation cycle, the 8 psi gas pressure on the primary side 22 of isolator 18 drives membrane 20 toward the secondary side 24, forcing the gas in secondary side 24 into extender 14. Because of its small diameter, however, catheter 12 acts as a constriction to the rapid flow of gas to balloon 10. Hence, when membrane 20 has moved fully forward (i.e., it hits the wall on secondary side 24), there is a relatively large pressure differential across catheter 12, and balloon 10 is only partially inflated. The process of balloon inflation continues as the gas in extender 14 flows through catheter 12 to the balloon until a state of equilibrium is reached in the closed portion of the system. It is therefore apparent that the pressure differential across catheter 12 is highest at the beginning of the inflation cycle and drops to zero at the end of the inflation cycle. Since the rate at which gas flows from extender 14 to balloon 10 is dependent upon the pressure differential across catheter 12, this gradual decay in the pressure differential results in a steadily decreasing flow rate and, therefore, a longer overall time until equilibrium is reached and the balloon is fully inflated.
A similar situation occurs during the deflation portion of the cycle. Thus, as the deflation cycle begins, a large negative pressure is created on primary side 22 of isolator 18 by negative pressure source 32. This negative pressure pulls membrane 20 toward primary side 22, whereupon the gas in extender 14 is drawn into the secondary side 24 of the isolator. Again, the small diameter of catheter 12 constricts the flow of gas out from balloon 10 such that, with membrane 20 moved to its fully retracted position (i.e., against the wall on primary side 22), a relatively large pressure differential exists across catheter 12, and balloon 10 is only partially deflated. As helium flows slowly from balloon 10 through catheter 12, the balloon continues to deflate until equilibrium is reached. Here again, the pressure differential across catheter 12 which drives balloon deflation is at its highest at the beginning of the deflation cycle and drops to zero at the end of the cycle. The gradual decrease in the pressure differential results in a steadily decreasing flow rate across catheter 12, lengthening the overall time until the balloon is fully deflated.
At first blush, it would appear that more rapid inflation/deflation cycles can be achieved simply by using a higher positive pressure during inflation and a lower negative pressure during deflation. The use of a higher positive pressure, however, creates the risk of overinflating and stressing the balloon, with the attendant risk of aneurization or rupturing of the balloon. Alternatively, simply increasing the volume of the isolator so that the maximum pressure differential across catheter 12 would be maintained for a longer period of time before membrane 20 has bottomed out would, without other modification to the system, create problems. Not only would there be a risk of damaging the balloon through overinflation, there would also be a need to remove a larger amount of gas from the balloon during deflation, which requirement would increase the deflation time.
There are generally three aspects of the operation of intra-aortic balloon pumps which contribute to inflation/deflation cycle times--the time required to deliver electrical signals from the controller to the various valves; the time required to effect the mechanical operations, i.e., movement of the isolator membrane and actuation of the valves between open and closed positions; and the time required to move the gas, either between the positive and negative pressure sources and the isolator on the primary side, or between the balloon and the isolator on the secondary side. By reducing the time needed to perform any one of these operations, more rapid inflation/deflation cycles may be achieved.
One approach for increasing inflation and deflation speeds by reducing gas movement time is shown schematically in FIG. 2 and described in U.S. Pat. Nos. 4,794,910; 4,796,606; 4,832,005; 5,158,529 and 5,169,379. In this approach, a valve 25 is positioned between the secondary side 24 of isolator 18 and extender 14 so as to separate the reusable drive unit from the disposable components. Valve 25 isolates the balloon 10, catheter 12 and extender 14 from isolator 18, thereby enabling the secondary side 24 of isolator 18 to be pressurized before balloon 10 needs to be inflated, and to be depressurized before balloon 10 needs to be deflated.
In the operation of the system of FIG. 2, an inflation cycle is initiated by closing valve 25 and opening valve 30, causing membrane 20 to move toward and pressurize secondary side 24 of isolator 18. Since valve 25 is closed, no helium flows toward balloon 10 which remains in a deflated state. When inflation is required, valve 25 is opened, causing the pressurized helium in secondary side 24 to flow through extender 14 and catheter 12 to inflate balloon 10. Since secondary side 24 of isolator 18 is already pressurized at the time valve 25 is opened, inflation of balloon 10 occurs more rapidly than with the system of FIG. 1 in which secondary side 24 must first be pressurized when inflation is called for. Once balloon 10 has been inflated, valves 25 and 30 may be closed and valve 36 briefly opened to vent the gas from primary side 22, after which valve 36 is closed. With valve 25 still closed, valve 34 may be opened, whereupon a negative pressure is created in primary side 22, pulling membrane 20 toward primary side 22 and creating a negative pressure in secondary side 24. When deflation is desired, valve 25 may be opened, whereupon the helium is drawn out from the balloon. Since a negative pressure already exists on secondary side 24 of isolator 18 when the deflation cycle begins, balloon 10 deflates more rapidly than with the system of FIG. 1 in which a negative pressure must first be developed in secondary side 24 when deflation is called for.
Despite the more rapid inflation/deflation cycles attainable with the system of FIG. 2, still more improvements in cycle speeds are desirable. Faster inflation and deflation cycles would provide operational benefits, including improved operational reliability at high heart rates, increased augmentation of patient blood pressure, and improved tracking of the patient's heart activity in cases of arrhythmia. These improvements in response times preferably will be obtainable without the use of higher magnitude operating pressures and the risks of leakage and balloon failure attendant therewith.